Lead free solder joints characterisation using single lap shear tests

Abstract

The study aims at characterizing the mechanical behaviour of a lead free solder material at a representative scale. The specific alloy considered in this work is SnAg0.3Cu0.5. The test bench comprises a specific single lap shear specimen geometry with grooves associated to a contactless method of monitoring. This allows to test solder joints with a thickness representative of microelectronics applications with good thermal gradients during the reflow process and thus metallurgic properties similar to the ones get with BGA dimensions. The campaign consists in monotonic shear tests and creep tests at 3 different temperatures. An emphasis on the fabrication process and the monitoring data treatment is given in this paper as it can leads to non-consistent results if not managed with precautions due to the reduced scale of the joint. Once the microstructure has been validated, experimental data are used to identify a specific viscoplastic material behaviour. Indeed, the primary creep strain has been found to be significant regarding the common secondary creep rate. Primary creep strain accumulation in cyclic loads could lead to an error of estimation in future predictive calculations if only secondary creep strain was considered as for the former solder materials. It is highlighted by successive creep steps at various stress levels performed in this study. Cyclic hardening has also been investigated but turns to be not relevant to model this particular plastic behaviour. The final set of material coefficients is provided for a full creep behaviour combining primary and secondary creep states coupled with Young's moduli at −40, 25 and 125°C monotonic loads.